Syria’s al-Qaida wing, the al-Nusra Front, said Friday that it had detained a number of Syrian rebels trained by the U.S.

In an online statement, the militant group accused the U.S. of recruiting "forces from what it calls ‘the moderate opposition’ ” and putting them in a training program run by the CIA. The group warned others against taking part in the “American project.”

The United States has trained fewer than 60 Syrian fighters since May in an attempt to counter the Islamic State group. Washington calls these Syrian rebels, who have been screened to exclude extremist elements, the New Syrian Forces.

The Pentagon denied that any U.S.-trained fighters had been captured. In a briefing Friday, Marine Corps Brigadier General Kevin Killea, chief of staff of the Combined Joint Task Force-Operation Inherent Resolve, said, “We have zero indication or information that there are any new captured New Syrian Forces that we’ve trained.”

A rebel faction known as Division 30 accused al-Nusra Front fighters of abducting its leader and several other members earlier this week. The group's representative said reports that its fighters were Western-backed were most likely the reason behind the kidnappings.

White House spokesman Josh Earnest said he had seen reports about the abductions but could not confirm whether the captured rebels had been trained by the U.S.

Earlier Friday, the al-Nusra Front attacked several rebel groups in northern Syria, the groups and an organization monitoring the war said.

The Syrian Observatory for Human Rights, a British-based pro-opposition group that relies on sources inside Syria, said the clashes left at least 13 people on both sides dead.

The Observatory also said Friday that warplanes believed to belong to the U.S.-led coalition had bombed al-Nusra Front positions near Azaz, about 50 kilometers (30 miles) north of Aleppo city.

Aleppo and Damascus, Syria

The al-Nusra Front had targeted rebel groups in the Azaz area, the Observatory said, escalating tensions between rival insurgents near the Turkish border.

A representative for Division 30, which was one of the targeted rebel groups, said al-Nusra Front fighters had attacked the group's headquarters near Azaz about 4.30 a.m., killing five members.

The representative, who requested anonymity for safety reasons, declined to say whether his group was among those trained by the U.S., saying only: "The Division supports any side that helps Syria and the Syrians against Daesh," referring to the Arabic acronym for the Islamic State group.

U.S.-backed

Observatory Director Rami Abdel Rahman told the French news agency AFP that the al-Nusra Front launched the attack to seize weapons "given by Washington to the rebels" but that the group had not been able to enter Division 30's base as clashes continued during the day.

Residents rest on the ground near rubble in a damaged neighbourhood in Aleppo, Syria, July 30, 2015.

Abu al-Hassan Marea, a Syrian activist who lives in Turkey near the Syrian border, said the Division 30 group had received training in Turkey, but did not know whether it was by the Americans.

The al-Nusra Front has a track record of crushing rebel groups that have received support from Western states, including the Hazzm movement that collapsed earlier this year.

The clashes came a week after Washington and Ankara announced an agreement to provide air cover for Syrian rebels and jointly sweep Islamic State fighters from a strip of land along the Turkey-Syria border, with U.S. warplanes using bases in Turkey for airstrikes.

In initial airstrikes, Turkey struck several Islamic State targets in Syria last week. However, the military has focused most of its recent airstrikes on Kurdish fighters affiliated with the PKK in northern Iraq.

A Turkish Air Force AS-532AL Cougar helicopter takes off from Incirlik airbase in the southern city of Adana, Turkey, July 27, 2015.

Partners in Syria

The Obama administration has long struggled to find partners on the ground in Syria to work with in its war against the Islamic State group.

The program for training moderate Syrian rebels to fight the Islamic State group has been faltering.

Pentagon officials said earlier this month that the vetting had been so strict that of an estimated 6,000 Syrian volunteers, only 1,500 were declared qualified so far, and that of those, fewer than 100 have been retained in the training taking place at bases in Jordan and Turkey.

More than 230,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests that descended into a civil war after a regime crackdown.